1. Field of the Invention
This invention relates to a light-emitting diode array, and more particularly to a heterojunction light-emitting diode array having a multiplicity of light-emitting diodes closely arrayed on a single substrate.
2. Description of the Prior Art
A light-emitting diode (LED) array consists of multiple p-n or p-i-n junction LEDs fabricated in a single substrate. An advantage of a light-emitting diode array is that it can be used to process image information with relative ease, by electrically controlling the discrete diodes. Because of this, light-emitting diode arrays are being improved and applied in a variety of ways.
With the increasing importance of electronic information in today's world, printers need to be able to print faster and at higher densities in order to cope not only with the growing amounts of information, but also with the inclusion of image information in the form of graphs, drawings, photographs and the like. One way of achieving this is to use light-emitting diode arrays as the light sources in printers.
Laser printers, which employ a laser light source, and LED printers, which employ a LED array as the light source, are two examples of non-impact optical printers. A laser printer requires the use of a mechanical mechanism such as a rotating polygonal mirror for scanning laser beam, and a correspondingly complex optical system. An LED printer, on the other hand, only requires a drive system for electrically controlling the discrete diodes (hereinafter also referred to as light-emitting elements) that make up the light-emitting diode array. As an LED printer is therefore structurally simple and straightforward, requiring no mechanical moving parts, using instead an optically magnifying lens array, it is possible for LED printers to be smaller, faster and more reliable than laser printers.
FIG. 5 is a cross-sectional illustration of a conventional homojunction type LED array used in an LED printer. For simplicity only two light-emitting elements are shown. With reference to the figure, each light-emitting element is formed by the use of vapor-phase epitaxy (VPE) to deposit an n-GaAsP layer 414 (about 50 microns thick) on an n-GaAs substrate 110, followed by a SiN masking layer 18 and a diffusion of Zn to form Zn diffused regions 420 (each about 1.5 microns thick). The light-emitting element is formed by the p-n junction at the interface between the n-GaAsp layer 414 and the Zn diffused regions 420.
p-Electrodes 22 and an n-electrode 24 are then formed, followed by the application of an antireflection SiN layer 426. The SiN layer 426 is then removed from the non-light-emitting element portions to form a p-electrode 22 bonding pad.
The light-emitting diode array thus formed contains numerous lattice defects due to a lack of lattice matching between the GaAsP layer 414 used as the light-emitting material and the GaAs substrate 110. As a result there is considerable non-uniformity of the material itself, so the emission efficiency is low. In addition, because the p-n junction is a homojunction having a low injection efficiency, it is difficult to improve the emission efficiency.
The AlGaAs single heterojunction type light-emitting diode array shown in FIG. 6 was developed previously.
With reference to FIG. 6, liquid-phase epitaxy (LPE) is used to form a p-Al.sub.0.2 Ga.sub.0.8 As layer 514, an n-Al.sub.0.5 Ga.sub.0.5 As layer 520 and an n.sup.+ -GaAs layer 521 on a p-GaAs substrate 310. n-Electrodes 322 and a p-electrode 324 are then formed by deposition, and the unnecessary portions of the n-electrode 322 are removed using photo-lithography and plasma etching.
A chemical process is then used to selectively etch the n.sup.+ -GaAs layer 521. Photolithography and chemical etching are then used on the non-light-emitting region to form the n-AlGaAs layer 520 penetrating about 1 micron into the p-AlGaAs layer 514. Plasma CVD is then used to form an antireflection SiN layer 426, followed by the use of heat treatment to form ohmic contacts for the n-electrode 322 and p-electrode 324, thereby completing the fabrication of the heterojunction light-emitting diode array.
Structurally, this type of heterojunction light-emitting diode array consists of conventional individual high-luminance LEDs arranged into a single array. The use of a heterojunction provides an improvement in the injection efficiency, and by using an n-AlGaAs layer 520 which is transparent to the 720 nm light emitted by the light-emitting p-AlGaAs layer 514, energy attenuation caused by internal absorption is avoided, enabling an emission efficiency to be achieved that is several times higher than that achievable with the light-emitting diode array shown in FIG. 5.
However, there are a number of problems with the above-described LED arrays. As is known, unlike when single discrete LEDs are involved, in an array of LEDs consisting of a multiplicity of light-emitting elements closely arranged on a single substrate, it is important to achieve reduced optical crosstalk between light-emitting elements to prevent deterioration of characteristics caused by reflection or diffusion between the elements of adjacent light-emitting elements or at the edge portions of the SiN dielectric layer.
As described above, conventional heterojunction type light-emitting diode arrays use the n-AlGaAs layer 520 as a transparent window to raise the emission efficiency. It therefore follows that reducing the crosstalk between adjacent elements requires the use of an etching process for complete removal of the n-AlGaAs layer 520 between elements. Also, to reduce optical bleeding, non-mesa portions of the emission layer have to be etched to a certain minimum depth.
It is known that the diffusion length of minority carrier electrons injected into the emission layer from the heterojunction is in the order of 10 microns. Therefore the emission layer has to be etched to at least 10 microns in order to reduce optical bleeding and attain optimum emission efficiency, but because of the difficulty of obtaining uniform etching with good reproducibility, the result has been a tendency towards a deterioration in device characteristics.
One solution, which is employed in the case of discrete LEDs, is to incorporate a double heterostructure by providing an AlGaAs layer with a high Al ratio below the AlGaAs emission layer. However, as mentioned above, when the arrangement is an array constituted of a multiplicity of light-emitting diodes consideration must be given to the possibility of crosstalk occurring between adjacent light-emitting elements. As such, deep etching is required to isolate the AlGaAs window layer, and the large increase in non-uniformity this produces because a problem.